The present invention is generally directed to a signal frequency modulator and its method of control. More particularly, the present invention is directed to a video signal frequency modulator and a method for its control the in which carrier and deviation frequencies are automatically controlled.
In general, a video cassette recorder separates a video signal into luminance and chrominance signals, and modulates them. The luminance signal is modulated to a frequency determined by the recording format, such as VHS or 8 mm. For instance, if an NTSC video signal is to be recorded in a VHS format, the luminance signal is modulated to a frequency of 4.4 MHz at white peak level and to frequency of 3.4 MHz at sync tip (i.e., blanking) level, thereby having 1 MHz of frequency deviation.
FIG. 1 is a circuit diagram of a conventional video signal frequency modulator. The conventional video luminance signal frequency modulator is equipped with clamping circuit 91, buffer 92, V/I (voltage to current) converter 93, and oscillator 94, frequency-modulating the video signal. Clamping circuit 91 receives the video luminance signal from an AGC amplifier (not shown) and clamps the signal at the sync tip level. Buffer 92 sends the output of clamping circuit 91 to V/I converter 93. V/I converter 93 transforms the voltage change of the video signal into a current change. Oscillator 94 receives and frequency-modulates the output of V/I converter 93. Oscillator 94 may be easily implemented in the form of a monostable multivibrator. V/I converter 93 is composed of variable resistors VR1 and VR2, PNP transistor Q1 and bias-voltage supply source V.sub.REF.
Video luminance signal Y is fed to V/I converter 93 via clamping circuit 91 and buffer 92. The voltage change of the video luminance signal, input to the emitter of transistor Q1 via variable resistor VR2 of V/I converter 93, is converted into a change of collector current I.sub.in of transistor Q1, and is output to oscillator 94. Bias-voltage supply source VREF is connected to the base of transistor Q1. Oscillator 94 frequency-modulates the magnitude of input current I.sub.in. The variable resistors VR1 and VR2 are controlled to adjust I.sub.in so that the modulated output of oscillator 94 will be suitable for the desired recording format. The carrier frequency is set by controlling variable resistor VR1 while observing the waveform of the frequency-modulated output FM VIDEO, and the frequency deviation is set by controlling variable resistor VR2. This controlling process is typically performed manually during the assembly of VCRs, cam corders, and other devices employing a frequency modulator. This is an obstacle to the automation of the assembly process. Further, the manual nature of the controlling operation makes it difficult to uniformly maintain the quality of products. To overcome such problems, the invention described U.S. Pat. No. 4,918,529 was proposed. In the disclosed device, a white peak level signal and sync tip signal are received from an insertion circuit according to control signals and are frequency-modulated, and the frequency-modulated signal is compared with a reference signal so as to automatically control the gain of a variable amplifier and the output of an adder. Despite the advantage of automatically controlling the video signal frequency modulator, the disclosed device requires additional analog circuits for a frequency generator and discriminator.